In
a small population, matings between relatives are common. This inbreeding may
lower the population's ability to survive and reproduce, a phenomenon
called inbreeding
depression. For example, a population of 40 adders (Vipera berus, shown at right)
experienced inbreeding depression when farming activities in Sweden isolated
them from other adder populations. Higher proportions of
stillborn and deformed offspring were born in the isolated population
than in the larger populations. When researchers introduced adders from
other populations  an example of outbreeding  the
isolated population recovered and produced a higher proportion of viable
offspring.

The explanation for inbreeding depression lies in the evolutionary history of the population. Over time, natural selection weeds deleterious alleles out of a population  when the dominant deleterious alleles are expressed, they lower the carrier's fitness, and fewer copies wind up in the next generation. But recessive deleterious alleles are "hidden" from natural selection by their dominant non-deleterious counterparts. An individual carrying a single recessive deleterious allele will be healthy and can easily pass the deleterious allele into the next generation.

When the population is large, this is generally not a problem  the population
may carry many recessive deleterious alleles, but they are rarely expressed.
However, when the population becomes small, close relatives end up mating
with one another, and those relatives likely carry the same recessive
deleterious alleles. When the relatives mate, the offspring may inherit two copies
of the same recessive deleterious allele and suffer the consequences of
expressing the deleterious allele, as shown in the example below. In the
case of the Swedish adders, that meant stillborn
offspring and deformities.

For Swedish adders, the solution to the inbreeding depression problem was simpleintroduce adders from other populations. But if the northern hairy-nosed wombat suffers from inbreeding depression, there are no other populations that can rescue it. Understanding the evolutionary history of a population and the likelihood that it carries recessive deleterious alleles, suggests that we should not allow population sizes to dip too low in our conservation efforts, or inbreeding depression may jeopardize the survival of the species.